For many years, it has been realized that successful explosives are based on a nitrate compound. This realization is based on the recognition that the success of an explosive depends on the very rapid oxidation of a fuel to produce gaseous products, e.g., carbon dioxide, water vapor, etc. In gunpowder, for example, carbon and sulphur provide the fuels, and potassium nitrate (saltpetre) provides the oxygen necessary to oxidize them. If the potassium nitrate was omitted, the carbon and sulphur would necessarily depend on external atmospheric oxygen, and would only burn at the outside surface, the flames spreading slowly through the mass, creating no discernible explosive effect.
Gunpowder is a heterogeneous mixture of the three constituents, and the rate of burning is dependent on the grain size, since the interior of each grain is shielded from oxidation. It was, therefore, realized that, if the nitrate radical could be attached to a fuel at the molecular level, the oxidation would proceed much more quickly, producing the class of substances known as "high explosives".
The source of oxygen for explosives requires that the oxygen should be stably bound at the highest expected storage temperatures, but should be available instantly at flame temperatures. So far, only the nitrate radical has been found to have these properties. Radicals, such as sulphate, or sulphite, are too firmly bound to dissociate and release oxygen at flame temperatures.
Therefore, one way to detect concealed explosives is to sense the presence of nitrogen, or, more preferably, nitrogen and oxygen in larger quantities than normally expected in the luggage of air traffic travelers. The probe used to detect these elements must be both very penetrating and element-sensitive. In practice, this limits the available probes to uncharged particles, either photons (gamma rays) or neutrons. Many Patents have been issued on the use of both of these probes, most of which are expired, while recent installation of commercial explosive protection equipment at airports has produced newspaper and other articles about the principles of operation.
So far, the prior art processes used to detect concealed explosives have irradiated suspicious containers with thermal or fast neutrons, and thereafter have looked for emitted gamma rays produced by the absorption of a neutron by the nitrogen nucleus and the subsequent decay of the nitrogen-15 daughter nucleus. An alternative prior art method has been to irradiate the container with gamma rays of the correct energy to excite an energy level in the nitrogen-14 nucleus. This excited nitrogen nucleus promptly returns to its lowest energy state, emitting a cascade of gamma rays in the process. Detecting this cascade, once again, signals the presence of nitrogen. Both of these prior art processes can be adapted to look for characteristic gamma rays of oxygen, and, in principle, enable the detection of unexpected nitrates in the containers or luggage.
For example, U.S. Pat. No. 3,146,349 to Jordan, discloses a method of detecting explosives in containers in which the explosive is seeded with a neutron absorber material (boron), and, when the luggage is irradiate with thermal neutrons, any gamma rays directly emitted by the absorber boron material are detected by suitable detectors.
In U.S. Pat. No. 3,832,565 to Bartko, an explosive detection apparatus is provided, which produces thermal or fast neutrons that irradiate the luggage and subsequently produce the emission of gamma rays, in response to the presence of nitrogen in the luggage.
In U.S. Pat. No. 3,594,577 to Loveday, another gamma radiation indicating or detecting apparatus is disclosed.
In U.S. Pat. No. 3,308,296 to Cowan, a hidden explosive detector is provided, which detects gamma ray cascades emitted from a long-life low activity radioactive tracer, which as been seeded into the explosive.
In U.S. Pat. No. 3,124,679 to Tittmen et al, a method and apparatus is disclosed for detecting the presence of a selected element by irradiating the element with thermal or fast neutrons and observing the emitted gamma rays produced by the absorption of a neutron by the element to be detected.
In U.S. Pat. No. 4,864,142 to Gomberg, a method and apparatus is disclosed for detecting the presence of a preselected element (contraband) in an object such as a suitcase. In this particular system a neutron beam is directed into the object being interrogated and the resonantly scattered neutrons by the preselected element are measured.